Flexible risers and dynamic umbilicals are being used increasingly in harsh environments which require reliable prediction of fatigue life. Predicting fatigue life of flexible risers and umbilicals are particularly challenging because of layered construction and helical components such as tensile armors and steel tubes. The basis for fatigue damage assessment of such elements is the long-term stress cycle distribution at critical locations on the helix elements caused by long-term environmental loading on the system. Establishment of the long-term stress cycle distribution requires global dynamic time domain analysis followed by cross-sectional analysis in a large number of irregular sea states.
An efficient fatigue analysis scheme is outlined in this paper. A detailed description is given of cross-sectional analysis techniques required for fatigue stress calculation in helix elements subjected to bending, tensile and pressure loading. Special emphasis is placed on assessment of friction stresses caused by the stick/slip behavior of helix elements in bending. These effects are considered to be of special significance for fatigue life calculations of deep water applications of flexible pipes and umbilicals due to increased contact pressure between the individual elements in the cross-section. A Joint Industry Project (JIP) with an objective of validating the stress calculation methodology by comparing with high quality bending tests has been initiated. Details of qualification testing for validating the methodology are also presented in the paper.